IMS 2013: 5 hurdles in high-speed and microwave

From cross talk to power integrity--high-speed and microwave designers air shared challenges in IMS workshop.

"The two groups--high speed and microwave designers--never seem to get together and talk to each other," said Ken Wong, senior marketing manager with Agilent Technologies, on Monday at the opening of the High Speed Signal Integrity Workshop at IEEE's International Microwave Symposium (IMS 2013) in Seattle, Washington.

To help rectify this problem, Wong, Heidi Barnes and Mike Resso, all from Agilent Technologies, coordinated a one-day signal-integrity workshop at the 2013 IMS Conference, with invited experts from around the world presenting one-hour sessions. The line-up included:

Jack Carrel, from Xilinx, talked about three techniques to remove fixture effects from 28-Gbps transceiver measurements.

Don DeGroot, from CCN Labs, talked about analyzing the loss in channels and the challenges of separating conductor and dielectric loss.

Xin Chang and Leung Tsang, from University of Washington in Seattle, talked about new simulation techniques to rapidly analyze via models and the impact on loss from the surface roughness of copper.

Marco Garelli, of HFE, talked about new, faster calibration procedures that can also quantify the errors in VNA measurements.

And I, Eric Bogatin from Bogatin Enterprises, talked about two specific cases of pathological crosstalk that can ruin any product at 28 Gbps.

At 28 Gbps, with a Nyquist at 14 GHz, signals are well into the microwave regime. "There's a lot of value for signal integrity engineers to leverage the tools and techniques used in the microwave world," Barnes said. "For example, while high-speed digital performance is defined in the time domain, the frequency domain world of the RF engineer is a powerful tool for measurement and analysis in signal integrity applications."

Two important differences distinguish signal integrity and microwave applications. The bandwidth of digital signals extend from DC to above 10 GHz, compared with typically narrow-band RF and microwave carrier signals. While many RF and microwave applications can be implemented on a double-sided substrate, digital applications require four to more than 40 layer boards.

"The answer I get from microwave engineers to address some of the transmission-line design challenges they hear about in high-speed digital designs is to use coplanar wave guides," Don DeGroot said. "They just don't get it."

From the transmitter to the receiver are all the interconnects. Jack Carrel calls this the "high-speed interconnect ecosystem, where each structure potentially interacts with every other structure through losses, reflections and cross talk."

Barnes offered five problems created by the interconnect, any one of which can prevent a successful product at 10 Gbps and above: losses, reflection noise, cross talk, skew and power integrity. As supply-voltage levels decrease and power-supply currents increase, the inductive noise in the low-impedance power paths is becoming just as big a problem as signal integrity in the controlled-impedance signal channels.

The high-speed interconnect ecosystem and these five growing problems are illustrated in Figure 2.

Click on image to enlarge.

Figure 2. The high-speed interconnect ecosystem extends from the transmitter to the receiver and all the interconnect in between. Along the way, five problems can cause system failures. Courtesy of Heidi Barnes, Agilent Technologies.

With more workshops like this one, microwave engineers may learn to treat their cousins in the high-speed digital world with a little more respect.

Eric Bogatin is a signal integrity evangelist with Bogatin Enterprises, where he teaches advanced signal integrity classes worldwide. He has a BS degree in physics from MIT, and MS and PhD degrees in physics from the University of Arizona in Tucson.. He writes the blog, Be the Signal. For more about Eric Bogatin, click here.